A solder mask is a hard, permanent layer of non-conductive material which covers the surface of a printed circuit board or the like, encapsulating the traces of the printed circuit itself. By solder mask is meant herein a hard, permanent layer which meets at least the minimal requirements of the abrasion resistance tests as defined in IPC-SM-840B, Table 12, Summary of Criteria for Qualification/Conformance (Institute for Interconnecting and Packaging Electronic Circuits). The solder mask is patterned to fully cover the circuitry, except for those portions intended to be exposed, e.g., for soldering to another component. Solder masks are typically formed from a layer of photoimageable composition which is applied to a surface of the printed circuit board. The photoimageable layer is exposed to actinic radiation which is patterned by means of a template or artwork. Subsequent to exposure, the photoimageable layer is developed in an organic solvent or an aqueous or semi-aqueous solution which washes away either exposed or unexposed portions of the layer (depending upon whether the photoimageable material is positive acting or negative acting). The portion of the layer which remains on the surface is then cured, e.g. with heat and/or UV light, to form a hard, permanent solder mask intended to protect the printed circuitry for the life of the board.
One prior art method of applying the layer of photoimage composition to the circuit board surface is to apply the material in liquid form, and then either allow it to dry or partially cure the material to form a semi-stable layer.
There are a number of advantages to applying a photoimageable layer to a circuit board as a dry film rather than as a liquid. In particular, dry films are free of organic solvent and therefore eliminate this hazard from the workplace and eliminate the need for apparatus to protect the immediate work environment and the more general environment from organic solvent emissions. Typically, a dry film comprises a cover sheet of support material which is somewhat flexible but which has sufficient rigidity to provide structure to a layer of photoimageable composition which overlies one surface of the cover sheet. Typically, the cover sheet is formed of polyester material, such as polyethylene terephthalate (PET), such as that sold as MELINEX.RTM..
To protect the photoimageable layer and to enable the dry film to be rolled, it is conventional that the exposed surface of the photoimageable layer be covered with a removable protective sheet e.g., a sheet of polyethylene. An example of such a dry film is sold as LAMINAR DM.RTM. by Morton International, Inc.
The method of use of such a prior art dry film is generally as follows. The protective sheet is removed from the photoimageable composition layer immediately prior to application of the dry film to the surface of the printed circuit board. This may be accomplished, for example, using automated apparatus which peels away and rolls up the protective sheet as the dry film is unrolled from a reel. The dry film is applied to the surface of the circuit board with the photoimageable layer in direct contact with the board surface. Using heat, vacuum and mechanical pressure, the photoimageable layer is immediately laminated to the surface of the board. The cover sheet remains overlying the photoimageable layer, protecting the photoimageable layer from exposure to oxygen and from handling damage. The cover sheet also permits an artwork (or template) to be laid directly on top of the dry film for contact printing, if contact printing is to be used (as is usually preferred from the standpoint of obtaining optimal image resolution). The dry film is exposed to patterned actinic radiation through the PET cover sheet. At this time, the PET support sheet is removed, permitting access to the exposed photoimageable layer by developer. Depending upon the composition of the photoimageable layer, the photoimageable layer is developed with organic solvent alkaline aqueous developer, or alkaline semi-aqueous developer. By semi-aqueous developer is meant herein a developer which is about 90% or more by volume alkaline aqueous solution, balance an organic solvent, such as 2-butoxy ethanol and other glycol ethers. The photoimageable layer may either be positive acting, in which case the exposed portions are removed by developer, or negative acting, in which case the unexposed portions are removed by developer. Most photoimageable layers for preparing solder masks are negative acting. Most photoimageable composition layers require some cure subsequent to development to render the layer hard and permanent so as to serve as a solder mask. Depending upon the composition of the photoimageable layer, curing may be effected with heat and/or UV light.
Printed circuit boards almost invariably have uneven surfaces in which circuitry traces are elevated over the surface of a board of non-conducting material. Circuitry traces may be the residual portions of an etched metal layer or may be built up from the board surface. It is desirable that a solder mask, particularly one formed from a photoimageable composition, conform to the contours of a circuit board surface. A conforming solder mask which adequately covers both the board surface and upstanding traces minimizes the use of expensive photoimageable composition.
U.S. Pat. No. 4,127,436 issued to Friel describes a process which produces a conforming solder mask over a printed circuit board. This patent defines the invention as:
"a process of applying a photoresist-forming layer to a surface having a raised areas, comprising:
(1) positioning the surface of a solid, unexposed, photoresist-forming layer adjacent to a surface having raised areas, while the other surface of the layer has adhered thereto with low-to-moderate adherence a thin, flexible, polymeric film support, PA1 (2) reducing the absolute gas pressure to less than one atmosphere in the region between the surface having raised areas and the surface of the layer, and PA1 (3) applying pressure to the entire surface of the film support at once over the area of the layer adjacent to the surface having raised areas, whereby the photoresist-forming layer is forced into intimate contact with the surface having raised areas. PA1 (4) exposing the layer, imagewise, to actinic radiation, PA1 (5) stripping the film support from the resulting image-bearing layer, and PA1 (6) removing areas of the layer imagewise to form a resist image on the surface having raised areas."
The invention also provides a process for forming a photoresist on a surface having raised areas comprising, in addition to the steps above, in either order of steps (4) and (5):
The Friel process provides for exposure (step (4)) either before or after the film support is stripped (step (5)) from the photoimageable layer. Exposure through the film support has the disadvantage of reducing resolution and furthermore limits the conformability of the photoimageable layer to the uneven substrate surface. Although the Friel process contemplates stripping of the film support prior to exposure, it is inherent from the total description in the Friel patent that such stripping be immediately prior to exposure. Once the support film is stripped in the Friel process, there is no protection of the tacky photoimageable layer from contact with surfaces and no protection against oxygen inhibition. As the process is described in the Friel process, the board surface to which the dry film is applied is contacted with other surfaces; thus it is inherent that the support film is not stripped from the photoimageable layer immediately after the dry film is applied to the printed circuit board, even if steps (4) and (5) are reversed. Therefore, the support film still acts as a limitation to the conformability of the attached photoimageable layer. If steps (4) and (5) are reversed, the film support no longer acts as a barrier to resolution; however, little is gained in this respect because the exposed, tacky photoimageable layer cannot be contact printed. Any distance between the off-contact artwork and the photoimageable layer is a limitation on resolution.
The above identified U.S. Pat. applications, of which this application is a continuation-in-part, describe processes of applying a solder mask-forming photoimageable composition layer to a printed circuit board using a dry film in which an intermediate layer is interposed between the "support film" or "cover sheet" and the photoimageable layer. The intermediate layer is selectively more adherent to the photoimageable composition layer than to the cover sheet, allowing the cover sheet to be removed after the photoimageable layer is applied to a printed circuit board with the intermediate layer remaining on the photoimageable composition layer as a "top coat". The top coat is of non-tacky material and can be placed in contact with other surfaces, such as artwork for contact printing. The top coat also serves as an oxygen barrier, allowing the photoimageable composition layer to remain unexposed on the printed circuit board, after cover sheet removal, for some length of time. The use of dry film having the "intermediate layer" or "top coat" make possible the processes described in these applications. In each case there is provided a conforming step, e.g., conforming vacuum lamination, after removal of the cover sheet. Because the cover sheet is removed prior to the conforming step, better conformance, particularly when applying thin photoimageable composition layers onto boards with closely spaced traces, is achieved relative to the degree of conformance achieved in the Friel process in which conformance is hindered by the support film that remains on the photoresist layer during the conforming step. Better resolution is also achievable because the top coat may be directly contacted with artwork for contact printing (unlike the off-contact process required of the Friel process when support film removal precedes exposure), and because the top coat is much thinner than a cover sheet or support film and is therefore much less a deterrent to good resolution than a support film (as in the Friel process where exposure precedes support film removal).
The results of the processes described in the above-identified U.S. Pat. application have been most encouraging. However, difficulties have been encountered in attempting to adapt these processes to an in-line system. This is particularly true with respect to the utilization of existing vacuum laminating apparatus in an in-line process. Vacuum laminators represent a considerable capital expense, and owners of such laminators are often reluctant to replace existing laminators in order to experiment with new dry films and new processes.
It is a general object of the present invention to provide a method of forming a conformed solder mask on a printed circuit board which achieves all of the advantages obtained in the process described in the above-identified U.S. Pat. application but which in many instances is more adaptable for in-line systems.